DESCRIPTION: Rhizobium etli enters into a symbiosis with Phaseolus vulgaris. Studies with R. etli mutants have shown that the lipopolysaccharide (LPS) O-antigen of the microsymbiont is required for infection of the host. In recent years, it has been found that compounds exuded by the host trigger a structural modification of the O-antigen that can be detected with monoclonal antibodies. One of the goals of the proposed research is to identify the actual chemical change(s) in the modified structure. Wild-type LPS will be purified from cultures grown under the inducing conditions and its structure compared to LPS form uninduced wild type. The analyses will include sugar and lipid compositions and various types of mass spectral and nuclear magnetic resonance techniques. With a recently developed mutant screen, bacterial mutants that do not appear to undergo this modification have been isolated. The second goal is to use this screen to isolate more mutants of this type. Four mutants isolated in this way are also defective in host infection. A third goal is to analyze the structure of the LPS of two of these mutants to test whether they differ from the wild type LPS under noninducing conditions or only under inducing conditions. If the latter is the case, a given mutant is likely to be altered in an enzymatic activity regulated in response to the plant signal or altered in the signal transducing regulatory mechanism. Regardless of whether the difference is limited to inducing conditions, determining the structural difference is important because it would identify a type of structural specificity that has been difficult to uncover in the study of LPS functions in bacterial-host interactions. Three of the mutations found by the mutant screen are located within a relatively short stretch of DNA. Sequencing of this stretch will commence in order to define its structure, including the number of genes and their organization into transcriptional units. The importance of this work would be to establish the existence of structurally-specific roles of the LPS in a biological system that is relatively easy to study and manipulate, and which, because of the wealth of information accumulating about it worldwide, is becoming an important model in microbe-host interactions.